1. Field of the Invention
The present invention relates to a method of injection blow molding synthetic resin bellows products. Particularly, the invention relates to a method of injection blow molding plastic bellows products such as constant-velocity joint boots, steering boots and the like.
2. Technical Terms
The term, "ratio of parison" or "parison ratio", throughout the specification and claims is defined by the formula: {(D-A)/(D-d)}.times.100, wherein "D" is a diameter of the crest portion of the product, "d" is a diameter of the root portion of the same product, and "A" is an outer diameter of the parison. The outer diameter of the parison refers to an outer diameter of the parison crest portion, since in the present invention the crest portion of the parison (including a crest incline portion) is solely subject to blow molding.
3. Description of Related Art
Hitherto, a blow molding method has been utilized frequently for forming a synthetic resin material into a bellows product. However, it has been found that, in the course of blowing up the base material in the bellows-shaped mold, the crest portions of the bellows product to be formed are blown up at a high ratio as against the root portions of same, resulting thus in extreme reduction of crest portions in thickness in contrast to a general thickness of the resultant bellows product as a whole. For example, if the thus-formed bellows product is an actuator, it will be the case that, in a practical operation of the actuator, a negative pressure applied thereto buckles its crest portions which are thin for the above reason, as a result of which the actuator's expanding/contracting action strokes are reduced, or what is worse, the product will not function as an operable actuator. Further, where the product is used as a duct for an air conditioner, such thin crest portions will reduce the thermal insulation property of the duct.
A solution to these problems could be thought to be by providing an increased thickness of the parison to compensate for the thin crest portions of the bellows product, but other portions than the crest portions will be increased in thickness more than normally required. Thus, the base material will be wasted in fact and may not be used economically. In addition, the resultant product will lose the original elastic property of bellows.
Another solution could be assumed by using an injection blow molding method to form the bellows product. In the injection blow molding method, a molten resin is injected into a mold to form a tube-like bottomed parison which is then expanded while being surrounded by a blowing mold to form a product having a final shape. Thus, the material used encounters such antinomic aspect: In the injection process, it should preferably be low in viscosity with good fluidity, while by contrast, in the blowing process, it should preferably be high in viscosity. Normally the material used is selected in preference to the blowing process, and therefore will not easily flow during the injection molding. It is thus common practice to form the parison into a tube-like shape having a uniform thickness and low flow resistance. The parison is entirely expanded along a bellows-like cavity formed in a blow molding outer mold to form a bellows-like barrel portion.
However, when a bellows product is formed by using such a tube-like parison having a uniform thickness, the blow-up ratio in the crest portions is locally different from that of the root portions, and a product with large variations in the thickness is finally obtained.
Particularly, with regard to a conical bellows product, such as a constant-velocity joint boot used in an automobile, in which its crest and root portions gradually increase in diameter as they proceed toward one end of the product, one can contemplate making the thickness uniform of the resultant product by forming a parison into a tapered shaft and widening the dimensions of the product progressively toward its one end. This becomes similar to open pressure molding, however, thus resulting in bubble creation through gas components in the resin used, or resulting in an intricate flow for the parison, so that internal resistance may easily remain in the parison. Consequently, problems occur such as sink marks, bubbles and voids in the parison per se. Even if one suggests forging an elongated bellows product, the molding will still be close to open pressure molding, which therefore raises the same problems such as sink marks, bubbles and voids in a parison. For these reasons, the injection blow molding method has been deemed incapable of forming a bellows product of uniform thickness, especially of forming a conical bellows product whose diameter becomes greater as it proceeds to one end of the product.
Those problems yet to be solved as to non-uniform thickness may seemingly be addressed by a technique for making the thickness of the parison itself non-uniform, which is proposed in the Japanese Utility Model Laid-Open Pub. No. 2-34211. But, such mere control of parison thickness has still encountered difficulty for molding a product with uniform thickness, in view of the blow molding process involving the steps of expanding a parison while reducing its thickness to obtain a desired shape of product, which tends to cause non-uniformity in distribution of blowing pressures, or changes in temperature of resin used, etc., thus resulting in forming a resultant product with non-uniform thickness. The same difficulty is even amplified for an intricate shape of bellows products, in particular, for a constant-velocity joint boot having conical bellows, which is asymmetric in the angles of the inclined portions connecting the crest and root portions in its axial direction, which makes it difficult to uniformly expand the parison and thus non-uniformity will easily occur in its thickness. Further, an occurrence of such thickness non-uniformity in the root portions of the bellow product makes it unsuitable for the present particular molding method to the constant-velocity joint boot for automobiles.
Additionally, one can consider rendering the parison non-uniform, using a constant ratio among the crest, root and inclined portions, but even in such case, the parison ratio is still high and as such it is difficult to calculate its volume, as a result of which, if any volume error of the product is increased, the core of the product will require correction to compensate for the error.